A great variety of bulk materials of different grain sizes are known to be stored and transferred for processing from the storage containers into other containers, e.g., scales, volumetric measuring containers, intermediate storage containers, etc. These are entire installations with interposed conveyers, and it is obvious that the actual container is provided with a valve element, which must be opened to remove the bulk material from the container. Where the bulk materials are coarse-grained, e.g., coal, merely large rocks, etc., i.e., the bulk materials have a very good flowability, there are no difficulties in providing variable discharge openings and hence material flows by opening the valve elements more or less widely, even in the case of simple shut-off valve elements underneath the container outlet.
However, the removal of bulk materials from containers becomes problematic in the case of reduced flowability, e.g., due to high percentage of fines in the bulk material. Very fine owder, e.g., lead oxide, iron oxide, and metal oxides in general, but also mortars, cement, clays, etc., are especially critical. It is known that these fine powders, and sometimes even bulk materials with coarser grain size, flow out of the opened container outlet poorly, and tend to form bridges within the container. When the outlet tapers downwardly, the material bridges at least within the tapering container outlet.
To overcome this problem, rubber cushions have been installed at the sides of the lower zone of containers, which rubber cushions can be inflated to prevent bridge formation and to guarantee the outflow of the bulk material, when the outlet opening is cleared by the valve element. Another possibility to eliminate the bridge formation by the bulk material located in the lower zone of a container is to admit compressed air through pipe lances inserted transversely through the side walls into the container and attached to the walls, into which lances pulsating air is forced to destroy the bridges already built up in the bulk material.
A container outlet is usually closed by a valve element, which is mounted freely in such a way that is able to clear the outlet with a controlled opening size. These include eccentrically-mounted hinged doors, whose fulcrum points are on the sides along the edge of the container outlet, also centrally-mounted regulating flaps, whose pivot shafts are in the center of the material flow, as well as segment gates. Even when the bridge formed by the bulk material located in the container outlet is eliminated or avoided through the above-described measures, it is often seen as a disadvantage that despite the opening of the valve element, the bulk material fails to flow out, but instead shoots out suddenly in a partly-uncontrolled manner when a certain flap angle is reached, after which the material again becomes stuck in the outlet. These phenomena occur especially in the case of fine powders, e.g. metal oxides or cement. Therefore, paddle-wheel feeder-type sluices have been mounted in the container outlet, but these are mechanically expensive.
According to another solution, the container outlet was left open with a relatively large opening at its bottom, and a collector was installed below it at a distance, so that the bulk material always drops directly onto the collector. However, in spite of sloping an elongated collector, the bulk material remains lying on it, because it has the observed poor flow properties. Therefore, shakers have been used, and the collector has been designed as a vibrating feeder. However, it is recognized that this also represents a considerable expense due to the necessary mechanical equipment.
The expense is not lower, either, when the collector under the open container outlet is a pneumatic conveying chute. The pneumatic conveying chute extends from the zone below the container outlet to the container to which the bulk material is fed, which is often a scale. It was possible to achieve a uniform material flow along the pneumatic conveying chute to the scales with moderate success in the case of the bulk materials that are not too fine. However, even pneumatic conveying chute fails to perform in the case of very fine materials, especially when conveying metal oxides, and it is not possible to achieve a uniform material flow even when vibrators are added. Moreover, the maintenance, operating, and investment costs of such collectors in addition to the containers are disadvantageously high. The installation dimensions are also unfavorable, because an installation thus equipped requires too much space.
Another disadvantage encountered in connection with the collectors was that the material flow is fed uncontrolled into the subsequent container, e.g., scales. In spite of the uniform intake of air into the container outlets provided with a lining permeable of gas, e.g., sintered metal or screen fabric, it happens frequently that the material shoots over the pneumatic conveying chute suddenly and in an uncontrolled manner. At any rate, it has not yet been possible to shut off the material flow in time and exactly when the filling height in the scales is reached in the case of the exact batching of a bulk material into subsequent scales. Therefore, one more valve element was installed at the end of the collector. However, this implies a further disadvantageous increase in the technical expenses.